Velocity modulated tube of the reflex type



1953 J. BERNIER ETAL VELOCITY MODULATED TUBE OF THE REFLEX TYPE Filed April 2'7, 1949 m. /a i a 1 I /7 2 i T 4 lln- 1 7r /5 I INVENTOR JEAN BERN/ER JEAN BROSSART BY M 9 AGENTS Patented Nov. 10, 1953 VELOCITY MODULATED TUBE O F THE REFLEX. TYPE- Jean Bernier and Jean Brossart, Paris, France, assignors to Compagnie Generale de Telegraphic Sans Fil, a corporation of France Application April 2'7, 1949, Serial No. 90,002 Claims priority, application FranceMay 5,1948

3 Claims. (01. 315-5) Our invention relates to velocity modulated tubes of the so-called reflex type, in which a socalled reflecting electrode, which is negatively biased with respect to the accelerating electrodes, reflects the velocity modulated electrons that have come out of the high-frequency field, so as to cause them to travel across said field in the opposite direction and suitably in phase to effect an absorption of energy from thebeam. These tubes only use one resonant cavity, which .simplifies construction and provides facilities for tuning and adjusting the circuits of the tube.

In such tubes, there exists in the space between the reflector and the electrode bounding the highfrequency field, a very substantial space charge that corresponds to substantially twice the cathode current. Owing to this space charge,the trajectories of the electrons are not straight, whereby the satisfactory operation of the tube and in particular its efliciency, is impaired. It

can in fact be seen, by considering Fig. 1 which I "that electrons belonging to different current streams b and c that flow in phase across the ;'high-frequency field in the initial direction have "unequal times of transit through the reflecting .--.space, so that the various electron current ele- :.ments are no longer in phase when they return 1 into the high-frequency fieldp This drawback particularly affects reflex klystrons, the cathode of which has a perveance,

of the cathode emission equation I =kV charge, the trajectories of the electrons remain substantially straight in the reflecting space and each electron follows substantially the same trajectory on the return travel as on the initial travel, so that the mean transit time of the electrons through the reflecting space remains very substantially the same in each of the trajectories.

The invention can also be advantageously used in systems, such as certain microscopes, wherein the electron optical system comprises mirrors; in

particular, it enables certain aberrations'to be g the return beam, and giving to the surfaces bounding said electrode, and also to' the surfaces of the anode and of the reflector,

The outer surface of they intermediateblec- I trode determines with the surface of the reflector *'which term is used to designate the coeflicient -ofthe order of 3, I0 being expressed in microrramperes and V0 in volts, producing an emission of 15 milliamperes at 300 volts or 94 milliam- :peres at 1000 volts, that is to say low/voltage or :medium power tubes, or the so-called wide electron tuning band tubes operating at high volt- :age, since in tubes of this type the reflecting tem of electrodes of a reflex tube according to .50

' that:

a practically uniform distribution of the field between said electrode and the reflector, and

The inner surface of the intermediate electrode determines with the surface of the anode a distribution of field between said electrode and the anode such that the electrons are kept along straight trajectories.

The shapes of the surfaces in question are calculated in a variable manner according to, the voltages applied to the intermediate electrode,

according to the shape ofthe beam, and the like.

The invention willbe more clearly understood, by considering the accompanying Figures 1 to 4, in which: 7 I

Figure 1 is a schematic view of a conventional klystron. H r v I Figure 2 shows a diagrammatic section in an axial plane, of a klystron provided with the improvements accordingto the invention.

Figure3 shows a modification of the klystron of Figure '2, in which the electron beam is hollow.

Figure 4 shows, like Figure-3 another modification of the klystron of'Figure 2 which allows some improvements in the operation of said klystron.

Fig. 2 shows a diagrammatic. sectionfof a systhe invention. This section may beconsiderecl for erample as an axial sectionof a system with a cylindro-conical beam of the shape of a body of revolution about the. axis 0y, but it. is to be understood that it couldalsobe considered as showing a meridian sectionof a system with a 'iFor wet-lily covered beam of the shape of a disc of revolution about the axis 03:, or a cross-section of a system with a flat beam and a prismatic cavity.

Fig. 2 shows a beam of electrons i, for example a conical beam, which successively passes across the resonant .cavity' 42 "and the anode 8"whichris raised "to "the acceleration potential :oi the selectrons, said beam being reflected by means of an electrode 3 raised to a negative potential with respect to the cathode 1. Thapointsof the space at which the electrons returnbackwards iorm an equipotential surface at zero potentialwhich is located between the e1ectmiiesi3 and-.8, and which may be considered as-a' virtual :cathode.

The space charge in the beam is thus that :pro-

duced by a current twice the strength-.of-the cathode current, it is the same as the space charge which would exist in a beam-of thesameshape, emitted by a cathode which is of theshape of the equipotential surface located in the position of the virtual cathodefbut which produces, con- "trary to a real .cathodefanelectric f eld which is .not zero atithe'leverthereof. According 'to the invention, between the electrodes'3 and Big arranged an intermediate-electrode 9, the locationpf wfhichcoincides with the region of iormationpffthe virtual cathode.

'fIhesystenl should bedimensioned as-regards distances 'and'potentialsapplied, in" such a mannor, that thefpoint pf formation of the virtual cath ode'corresponds to the desired time oftransit, and the electric "held in *the' region in *which there is no space charge between the virtual lnca'thode"andthe'reflector is -precisely equal to the field "subsisting at the level of I the virtual cathode. v "the trajectories of the electrons to 1 be fstraighti and forthef return trajectories to coin- "cij'de-'e};actly"with"the 'injtial trajectories,"it is necessarythat the equipotential surfaces within thefbeam taking into account the space charge,

" the portions-ofconcentric-spheres and that the trajectories of thefielectrons' be the radii-"of said spheres.

Preferably, "therefore, the l surface of the -reflector ,3 may be iven the shape of a spherical segmentwhichisconcentricwith-the orthogonal spheres of the trajectories of" theelectrons, and the outer' sur'f-ace of the electrode 'a-may begiven a shape parallel" to the electrode *3,- so that the electric field in the regionin-which" there is no space'charge-aridwhich islocated bEtWGEIl'B and 9 will be seen-item uniform, since the equipotential surfaces are spherical segments. The electrode dis-provided with'a circularhole-of a diameters-ub'stantia-lly equal to that-of thebearn at the level of its reflecting surface; this 5 hole may be covered, although this is not-essential, *with an 'open-mesh grid 4 of the "shape of a spherical segment which is concentrid'tQ -S. Said grid' 'hasno electron-optical lens'efiect -si-nce the shape of the electrodes andthe =-voltages have been chosen, a -hereinbefore =stated;"-'in such a manner that the-electric field on citheriside of the grid- 4 is"the-- same the purpose-of this grid is'- merelyto constrain the equi-potential-surface to be of spherical shape andtthereby prevent said-surface from: being deformed by-an overlap f the field through thehole ln fl; which-might lzproduceaflfault' in the optical properties of the i -mirror. :Neverthelss; as already stateithegrid r4 is'notessential. l

V .The anode 8 is2alsoprovided :with a hole for =thelelectrons t passcthrough; this 'hole is \prefthough 2 this is Shot essential,

with a spherical grid 5 which is at right angles to the trajectories of the electrons and which may serve to bound the high-frequency field of the cavity 2. It should be noted that since the electric fields on either side of the grid 5 are not .egmal, said grid forms .a ,lenswhich must be etalgeninto acqoimt for-.determiningaby the well known means, the angle of divergence of the gcone knowing the angle of incidence of the elec- 1 0 trons before they pass through the grid 5.

l t isthe vacuum tight envelope of the tube. In

said vacuum chamber is shown a cathode I heat- .edhyafilament 1. and a focusing electrode l5. El e reflector ,3lis raised to a negative potential zthrollghitheeconnection i8 and the intermediate electrode is raised through the connection l9 to a suitable potential.

.il'hezshapeoi the inner face of the electrode 9 and also the shape of the anode 3 are determined "brittle-condition that the distribution of the potential between 8 andfEi should be such that the f sforward andyreturn;trajectories of the electrons .coincideaand are straight.

Starting from this condition, it ispossible to .determinesaidshape for any shape of the beam, :.partly;-by j;C%1CLll@- ti ()l1 and partly by experiment. .Thetmethod of calculation may be based on ;the following procedure: the potential of the anodefi; the cathode current flowing through the :beam during the forward .travel, the diameter and ,theangle-of incidence of the beam at the level ,of .thegridli, and the distance between .the grids 4 and .5 whichis determined by the desired i time of a transit, are given. The density of '1theqelectron charge in the portion 6 of the 'beamiscalculatedand the electron-optical equations r latin tethepassage through the grid 5 .are written down, and also Poissons equation:

wherein so idcnotes thedielectric constant of the vacuum I in erationalized Giorgi iunits.

Thegra-phicon analyticsolution of these equa- -tions "makes.it possible todetermine the angl of divergence?- 20, I of 1-: the beam, the law of distribution ofzthe potentialzinside the; beam:-between theeelectrodesisaeand9, and the-electric field. at the level-.of :the virtual. cathode located at 4.

It isithen possible .toadetermine the radii of thesphericalsurfaces l-and 5, the diameter of athei hole in theselectrode?'9,:,and the radius and -thervoltage.of'the spherical electrode '3 which l-havektoibesuchthatthe fieldat ;the levelof the grid' :4.;i-n:.-thaspacei-3e9 without an charge is equal to the field-:hereinbeforedetermined at the'level ofthe virtual-cathode.

:Forxdeterminingthe shapeoftheanode; 8 and ofithe inneresurfa'ceoftheelectrode -9, it, is possible to use;;as known, experiment i an electrolytic tank carried out on an enlarged model; the beam; is represented by an insulating substance, and by deforming the electrodes represcntingathe oppositesurfaces ofsthe electrodes 28 anti s, attempts are made to -produce in. the electrclyte a 1 potential. distribution which coin- =cides, along '5 the: insulating "surface representing "the beam; 2 with the: distribution of the potential :obtainedeby .integratingPoissons equation; fiFig. 3 shows-a modificationof a-Jdetail of 'Fig. 2,: :i-n;- which:the beamjfi is hollow. "The elements 'OfT- be. 2. are repeated therein,.viz;thereflector 3, the anode 8 and the intermediate electrode 9. i'I'h rgridM of saidaelectrodeiil hasat the middle athereot-aa solidconicalportion; I I which, for the hollow region of the beam, performs the same focalizing function as the inner surface of the electrode 9 does for the outer regions of the beam.

Similarly, the anode 8 has at the middle thereof a recess l2, the shape of which is determined by means of the method hereinbefore explained.

Fig. 4 shows a modification of a detail of Figure 2 wherein the grids 4 and 5 of Fig. 2 are eliminated and the anode 8 is provided with thin noses 22 which, in addition to the non-negligible electronoptical eifect that they have, tend to decrease the overlap of the high-frequency field which is favored by the elimination of the grids 4 and 5 and prevent said field from disturbing the grouping of the electrons in the reflecting space.

The reflector 3, the electrode 9 and the anode 8 are shaped according to the rules set forth which also define the characteristics of the system as regards dimensions and potentials, in such a manner that the beam 6 is reflected at the level of the grid 4 and follows on its return travel the same straight trajectories as during its forward travel.

At 20 is shown the lead of the circuit into which the high-frequency energy is delivered by the klystron.

The invention is not restricted to the embodiments described or to the applications considered but on the contrary admits of any modification in said embodiments and applications which might be within reach of an expert.

What we claim is:

1. A velocity modulation electronic tube com prising a source of electrons to be emitted in a beam, a velocity modulation cavity resonator having inlet and outlet grids interposed across the path of the beam, means comprising a connection between a first source of voltage and said resonator to raise the latter to a positive potential relatively to the said source of electrons, the surface of the resonator in the neighborhood of the outlet grid being concave in the direction of the beam, a reflecting electrode located in the axis located inside the beam at a level where the crosssection of the beam is smallest, means comprising a connection between a second source of voltage and said reflecting electrode to polarize the latter negatively with respect to said source of electrons thereby to reflect the beam toward the resonator, an intermediate electrode placed between the resonator and reflecting electrode and having an aperture surrounding the axis of the beam, said aperture being located within the equipotential surface corresponding to the oathode voltage, said intermediate electrode being bounded on the one side by a surface generated by a circle concentric with the said circle generating the surface of the reflecting electrode, said surface of said intermediate electrode being homothetic to said surface of said reflecting electrode, and means comprising a connection between a third source of voltage and said intermediate electrode for raising the latter to a voltage approximately equal to that of said source of electrons.

2. A tube as claimed in claim 1, wherein the surface of said reflecting electrode and the surface of said intermediate electrode are spherical, and centered upon a point located inside the beam at a level where the cross-section of the beam is smallest, the aperture of said intermediate electrode being circular.

JEAN BERNIER. JEAN BROSSART.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,411,913 Pierce et al Dec. 3, 1946 2,416,714 Pierce Mar. 4, 1947 2,445,404 Mayo July 20, 1948 2,445,771 Fremlin July 27, 1948 2,460,332 Bowman-Manifold Feb. 1, 1949 

